The present invention relates to an anisotropically electroconductive adhesive used for forming an adhesively bonded structure for establishing electrical connection in electric and electronic circuit boards and the like as well as an adhesively bonded structure formed by using the adhesive.
It is well established in the prior art to use an anisotropically electroconductive adhesive for simultaneously obtaining adhesive bonding and electrical connection between a flexible printed circuit board, referred to as FPC hereinbelow, and a printed circuit board, referred to as PCB hereinbelow, between a FPC and a display panel such as a liquid-crystal display, referred to as LCD hereinbelow, plasma display, referred to as PDP hereinbelow, or electroluminescence display, referred to as EL hereinbelow, and the like, each circuit board and display unit being provided with an array of electrodes or terminals to be electrically connected with those in the counterpart.
It is very important in order to ensure high reliableness of the electric and electronic instruments to have a structure adhesively bonded together by using such an anisotropically electroconductive adhesive to give firm and reliable electrical connection between the terminal arrays. Several methods have been undertaken for the non-destructive testing of the electrical connection in the thus adhesively bonded structure including the conduction test, microscopic observation test of the deformation of the electroconductive particles dispersed in the adhesive matrix, measurement of the thickness of the adhesive layer and so on.
Since completeness of the electrical connection in the thus adhesively bonded structure heavily depends on the melting or curing condition of the adhesive resin, it is proposed that the anisotropically electroconductive adhesive is admixed with an irreversibly thermochromic pigment which serves to provide a means for obtaining the information on the highest temperature reached during the bonding works from the change in the color of the pigment.
None of the above described prior art methods, however, is satisfactory due to its own limitation. For example, the conduction test is applicable only when the assemblage work of the instrument has gone to a considerably high stage near to completion so that detection of incomplete electrical connection is sometimes too late to take a measure for remedy. The observation of the deformed conductive particles can be performed only by the use of a microscope. When the adhesive composition contains a heat-curable resinous ingredient, in particular, deformation of the conductive particles not always provides a direct indication of the condition of curing of the adhesive. The method of thickness measurement is an empirical method which is possible by knowing the statistical correlation between the thickness and probability of occurrence of incomplete connection so that reliableness of the method is sometimes unavoidably poor more or less. The method of using a thermochromic pigment is also defective because, while the thermochromic color change of the pigment takes place only as a result of the highest temperature which the temperature of the adhesive composition has reached regardless of the duration at such a temperature, curing of an adhesive composition is a result obtained only when the adhesive composition is kept at a certain temperature for a certain length of time. What is worse is that the thermochromic pigment in an adhesive composition sometimes acts as an impurity which may cause a decrease in the adhesive bonding strength.